Compare the three types of muscle histologically. Describe the structure of skeletal muscle and the sarcomere.

Types of Muscle

Skeletal Muscle — Structure

Cardiac Muscle

• Branching cells joined end to end by intercalated discs
• The disc contains three junctions — fascia adherens (anchors actin), desmosomes (mechanical), and gap junctions (electrical coupling)
• The gap junctions make the myocardium a functional syncytium, so it contracts as a whole — the histological basis of the all-or-none behaviour of the heart
• Purkinje fibres are modified cardiac muscle cells — larger, pale, glycogen-rich with few myofibrils, specialised for conduction rather than contraction
• Atrial myocytes contain granules of atrial natriuretic peptide

Smooth Muscle

• Spindle-shaped cells with a single central nucleus, which becomes corkscrew-shaped on contraction
• No sarcomeres and no striations; actin and myosin are arranged obliquely and anchored to dense bodies, the equivalent of Z lines
• Caveolae replace T tubules
• Gap junctions permit spread of excitation in visceral (single-unit) smooth muscle; multi-unit smooth muscle (iris, vas deferens) has few
• Capable of division and of secreting its own collagen and elastin — important in vascular remodelling and in the pregnant uterus

The Neuromuscular Junction and Motor Unit

• Motor end plate — the axon terminal loses its myelin, and lies in a synaptic gutter with junctional folds bearing nicotinic acetylcholine receptors
• Motor unit = one motor neurone and all the fibres it supplies; small in precise muscles (extraocular, about 1:5) and large in powerful ones (gluteus maximus, up to 1:2000)
• Muscle spindle — the stretch receptor, containing intrafusal fibres supplied by gamma efferents
• Golgi tendon organ — monitors tension, in the musculotendinous junction

Muscle Fibre Types

• Human muscles are mixed; the proportion is genetically set and modified by training
• Histochemical staining for myosin ATPase distinguishes them, and shows the normal chequerboard mosaic — whose loss (fibre type grouping) indicates reinnervation after a neuropathy

Applied Aspects

• Myasthenia gravis — autoantibodies against the nicotinic acetylcholine receptor; fatiguable weakness worse at the end of the day, with ptosis and diplopia. Improves with anticholinesterases
• Duchenne muscular dystrophy — absence of dystrophin, which links the cytoskeleton to the matrix; the fibres degenerate and are replaced by fat and fibrous tissue, giving pseudohypertrophy of the calves
• Cardiac muscle cannot regenerate — an infarct heals by fibrous scar, and function is permanently lost. This single histological fact explains why reperfusion within hours matters so much
• Rhabdomyolysis releases myoglobin, which is nephrotoxic; creatine kinase is markedly raised
• Muscle biopsy distinguishes myopathy (random fibre size variation, internal nuclei) from neuropathy (grouped atrophy and fibre type grouping)
• Malignant hyperthermia — a defect of the ryanodine receptor of the sarcoplasmic reticulum; volatile anaesthetics trigger uncontrolled calcium release, rigidity and a rapid rise in temperature
• Smooth muscle hypertrophy and hyperplasia in the pregnant uterus is among the most striking examples of growth in an adult tissue
• Leiomyoma is the benign tumour of smooth muscle and rhabdomyosarcoma the malignant tumour of skeletal muscle, commonest in children
• Cardiac troponins are the markers of myocardial injury, released as the sarcomere breaks down after infarction